Such fans are used particularly for cooling internal combustion engines. Fluid friction clutches used for this purpose must be able to provide the required level of cooling by changing the drive rate or fan speed to modify the amount of cooling air provided by the fan. The rotational speed is controlled by a temperature sensor as a function of the temperature of the cooling water or cooling air, or by virtue of centrifugal force as a function of engine rpm. The degree of filling and therefore the degree of coupling are thus controlled, resulting in a change of the rotational moment which can be transmitted by the clutch. Depending on the degree of filling, the slippage of the clutch will vary, said slippage being equal to the difference between the rotational speed of the drive and the rotation speed of the fan. The result is a "slippage power," which results in a rise in the temperature of the viscous fluid transmitting the rotational moment.
The same conditions also apply to a so-called fluid friction clutch with limited rotational moment. In this design, the clutch is arranged so that only a moment which corresponds to the maximum permissible fan rpm will be transmitted. With a further increase in engine speed, the fan rotational speed will not increase or will increase only slightly. In this case also, a "slipping power" results, imposing a corresponding thermal load on the clutch.
In order not to overheat the viscous liquid and to keep bearing temperature within acceptable limits, the heat resulting from slipping must be dissipated into the surrounding air. For this reason, clutches of this type are provided with appropriate cooling ribs which are located particularly on the front of the clutch or on the back of the clutch and on the circumference of the clutch housing.
In known embodiments, the cooling ribs on the front and back of the clutch are arranged radially. In a known fluid friction clutch with temperature-controlled filling regulation, the clutch is provided on the cover side with cooling ribs, some of which run radially and some of which are slightly curved. Both rib shapes have the disadvantage that they produce a significant secondary air flow which cuts down the main flow from the fan hub.
Measurements with such cooling fans have shown that the downstream flow rate of the secondary flow at the periphery of the clutch may be significantly above the rate of the flow produced by the fan itself. The resultant reduction of the flow at the fan hub leads to a reduction of the air delivery rate and the degree of efficiency of the fan, and to an increase in the noise level as well.
Cooling ribs located on the back of the clutch, running radially or predominantly radially, also produce an additional flow, but the disturbing influence in this case is much less, since the main flow is deflected by the internal combustion engine in any case, in a radial or semiaxial direction. Moreover, the air flow produced by the cooling ribs is throttled by structural elements in the clutch, especially the coupling flange.
The secondary air flow produced by the cooling ribs on the front of the clutch, in contrast to a fan without this kind of flow, leads to a decrease in efficiency of 10-15 percent and, with the same air flow, a 10-15 percent higher power requirement for the fan. The noise level increases approximately 3 dB (A).